Digital storage oscilloscope with simultaneous primary measurement and derived parameter display on common time axis and method therefor

ABSTRACT

A method for presenting information and a digital storage oscilloscope are disclosed in which primary measurements of a signal are performed and displayed. Parameters are also derived for the signal based upon the primary measurement data. These derived parameters are then also displayed as a function of time on the display, preferably with a common time axis. This enables the oscilloscope operator to correlate features found by reference to the derived parameters directly to the primary measurements of the signal. Moreover, since the data from the primary measurements are stored in the oscilloscope, multi parameter calculation and parameter recalculation can be performed.

RELATED APPLICATION

This application is a continuation of application Ser. No. 09/037,155filed Mar. 9, 1998, now, U.S. Pat. No. 6,195,617. The entire teachingsof the above application are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Oscilloscopes are one of the more common examples of electrical test andmeasurement equipment. This class of devices makes primary measurementsof time varying signals. In most instances, these measurements are ofthe signal's voltage as a function of time, even though the detectedvoltage may be indicative of some other measurement of interest such aselectrical current through a resistive element or temperature in thecase of a thermistor, in two arbitrary examples. The oscilloscope isuseful because it enables the visualization of the time varyingcharacter of signals, using a vertical axis representing level andhorizontal axis representing time.

The digital storage oscilloscope (DSO) is a subclass of oscilloscopes inwhich the time varying nature of the sampled signals is representeddigitally within the device. The main advantage is that non-simultaneoussignal events can be stored in the device for subsequent comparison.Additionally, parameters can be derived from the digital data of theprimary measurements, such as statistical features of the signals.

In the typical implementation, the DSO works by waiting for thesatisfaction of some trigger condition. When the trigger event isreceived, the primary measurements of the voltage, for example, of thesignal are made, and the resulting measurement data are stored in awaveform memory. Successive positions in the waveform memory hold thedigitized level of the signal at increasing time delays from thetriggering event. Under current technology, DSOs can capture signals ata rate of up to 8 gigasamples/second (GS/s) with waveform memories of upto 16 million storage locations.

Where oscilloscopes are time domain instruments, spectrum analyzers makeprimary measurements in the frequency domain. In the typicalconfiguration, these devices plot the magnitude of the signal energy asa function of frequency-signal magnitude or level being on the verticalaxis with the frequency on the horizontal axis. These devices typicallyoperate by scanning a very narrow notch-bandpass filter across thefrequency spectrum of interest and measuring energy in the frequencybins. In this way, spectrum analyzers are useful in identifying thespectral distribution of a given signal.

For certain signal analysis problems, however, oscilloscopes andspectrum analyzers are not well suited to the task. For example, whentrying to isolate infrequent anomalies in a signal such as that requiredfor digital/analog circuit analysis/debugging or when trying to identifytrends in signals such as when analyzing modulated signals, bothoscilloscopes and spectrum analyzers are less useful. Since spectrumanalyzers operate by scanning filters across the spectrum, any shortterm changes in the signals are lost; and in most oscilloscopes, suchinfrequent anomalies will scroll by on the display at a rate that is toofast for the operator to analyze. Only DSOs retain the relevantinformation on the signals, but in order to analyze it, the operatormust scan through long arrays of data to find events that may not bereadily apparent from the primary measurements alone.

In order to fill this gap, electronic counters such as modulation domainanalyzers have been developed. These devices operate by performingprimary measurements of the signal, i.e., detecting the signal'scrossing time of a set threshold, and then generating plots ofparameters derived from the primary measurements, such as the signal'sfrequency, phase, or time interval as a function of time. For example,the frequency/phase versus time analysis are very useful in analyzingfrequency-shift-key and phase-shift-key, respectively, modulatedtransmissions; time interval analysis is useful for analyzingpulse-width modulated signals.

SUMMARY OF THE INVENTION

While being somewhat useful in analyzing obvious, recurrent trends inmodulated signal transmission applications, modulation analyzers areless useful in identifying and tracing specific anomalies in thosesignals and addressing the wide range of signal analysis that would bedesired. Modulation domain analyzers do not enable the operator tocompare the derived parameters such as frequency/phase to the actualprimary measurement of the signals such as its level, e.g., voltage.Modulation analyzers do not store the primary measurements, simplycalculating the parameters on-the-fly. As a result, it is stilldifficult for the operator to find the highly infrequent anomalousevent. Moreover, there is no way of identifying the location of theanomalous event in terms of the actual signal, even if it can be found.Still further, modulation domain analyzers require that the threshold bepreset. Once a measurement is made, there is no way to select adifferent threshold. And, the number derived parameters that are offeredby the devices is typically limited to phase, frequency, or timeinterval versus time.

The present invention concerns a digital oscilloscope that has beenaugmented with capabilities to derive and display parameters based onits primary measurements. In the preferred embodiment, the oscilloscopeis a digital storage-type oscilloscope that measures a time varyingvoltage. The derived parameters are calculated from the primarymeasurements and simultaneously displayed with those measurements on thesame time axis. This enables the oscilloscope operator to correlatefeatures found by reference to the derived parameters directly to theprimary measurements of the signal. Moreover, since the derivedparameters are preferably calculated based upon stored data from themeasurements, a large spectrum of different parameters can be calculatedand different thresholds, for example, applied to the same data set.

In general, according to one aspect, the invention features a method forpresenting information on a digital oscilloscope. The method comprisesmaking primary measurements of a signal. In the typical instance, thiscomprises measuring the signal's voltage as a function of time, althoughthe voltage can be indicative of some other time varying phenomenon,e.g., current, acceleration, or any transduced signal. The data from theprimary measurement is then displayed as a function of time. In onetypical implementation, the horizontal axis of the display representstime. According to the invention, parameters of the signal are alsoderived based upon the primary measurement data. These derivedparameters are then also displayed as a function of time on the display.In the preferred implementation, the primary measurements and thederived parameters are displayed on a common display with a common timeaxis.

In specific embodiments, the derived time-varying parameters aregenerated based upon a number of operations. For example, the derivedparameters can be generated by comparing the primary measurement data toa threshold. Such an operation yields parameters such as the time overwhich the primary measurement falls below or rises above a threshold asa function of time.

Additional features in further implementations derive parameters byfirst identifying cycles in the signal and then calculating theparameters for each of these cycles. Such operations are useful when,for example, plotting the time between minima/maxima and minima/maximain previous or subsequent cycles, the time of local minima and/ormaxima, period, per-cycle frequency, rise time, cycle fall time,over-shoot/pre-shoot, change in period, or change in pulse width fromcycle-to-cycle.

In additional or alternative features, the derived parameters can begenerated based upon first identifying cycles in the signal and thencomparing the primary measurements in each cycle to a threshold. Theseoperations are useful when determining peak-to-peak variation inamplitude, duty factor, and change in duty factor from cycle-to-cycle,for example.

In still further implementations, the derived parameters can begenerated by first identifying cycles in the signal and then comparingeach cycle to an absolute time reference. This is useful incommunication systems to determine phase differences and timing errors.Similarly, the derived parameters can be generated by comparing theprimary measurement data to a reference clock. This is useful fordetermining phase differences between the measured signal/clock andtiming errors, for example.

In general, according to another aspect, the invention also features amethod of operation for a digital oscilloscope. The method comprisesrepeatedly sampling and digitizing a signal at predetermined timeintervals and storing the data generated by the digitization. Parametersare calculated from these primary measurements. The derived parametersare then displayed as a function of time on the oscilloscope. In thisway, time-based parameters are calculated directly from the digitalprimary measurement data.

In general according to still another aspect, the invention features adigital oscilloscope. The oscilloscope comprises at least onedigitization channel that performs primary measurements of a signal andgenerates data indicative of those measurements. A data processing unitis then used to derive parameters from the data. The primary measurementdata and the derived parameters are then plotted as a function of timeon the digital oscilloscope's display.

In specific embodiments, each digitization channel comprises asample-and-hold circuit that freezes the signal. An analog-to-digitalconverter then digitizes the signal. The resulting data are stored in awaveform memory. Thereafter, the data from the waveform memory istransferred to a local memory unit. The data processing unit accessesthis local memory to calculate the derived parameters that are storedback to the local memory after calculation.

In specific implementations, the local memory comprises arrays ofstorage locations. The data from the primary measurements and thederived parameters are stored in separate arrays but such that theprocessor can re-correlate the arrays to have an analogous temporalorganization. In other words, the data processing unit can correlatederived parameters at specific locations with the data from the primarymeasurements that gave rise to the derived parameters. Preferably, thearray holding the derived parameters is padded to facilitate scalingoperations, such as zoom operations, performed for signal display.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

In the drawings, like reference characters refer to the same partsthroughout the different views. The drawings are not necessarily toscale, emphasis instead being placed upon illustrating the principles ofthe invention. Of the drawings:

FIG. 1 is a perspective view of a digital storage oscilloscope;

FIG. 2 shows a conventional display of a digital storage oscilloscope;

FIG. 3 is a schematic view of a digital oscilloscope display accordingto the present invention in which a pulse width jitter parameter displayis generated;

FIG. 4 is a process diagram illustrating the steps performed by thedigital oscilloscope to generate threshold-based parameter displays;

FIG. 5 is a schematic view of a digital oscilloscope display accordingto the present invention in which a period jitter parameter display isgenerated;

FIG. 6 is a process diagram illustrating the steps performed by thedigital oscilloscope to generate cycle-based parameter displays;

FIG. 7 is a schematic view of a digital oscilloscope display accordingto the present invention in which a cycle-to-cycle jitter parameterdisplay is generated;

FIG. 8 is a schematic view of a digital oscilloscope display accordingto the present invention in which a duty cycle jitter parameter displayis generated;

FIG. 9 is a process diagram illustrating the steps performed by thedigital oscilloscope to generate threshold and cycle-based parameterdisplays;

FIG. 10 is a schematic view of a digital oscilloscope display accordingto the present invention in which an interval error jitter parameterdisplay is generated;

FIG. 11 is a process diagram illustrating the steps performed by thedigital oscilloscope to generate time and cycle-based parameterdisplays;

FIG. 12 is an block diagram illustrating the organization of anoscilloscope according to the present invention; and

FIG. 13 is a block diagram showing the mapping of the arrays of thelocal memory to the display and the padding performed in the derivedparameter array according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

FIG. 1 shows a digital storage oscilloscope (DSO) to which theprinciples of the present invention are applied. Generally, the DSO 100comprises a display 110, on which the time-dependent data is displayed.A key pad 112 controls the operation of the display 110 and triggercharacteristics, for example. An optional printer 114 provides aprintout of the display, raw data, or the status of the oscilloscope.Four analog signal input ports 114A-114D connect to probes (not shown).The probes are used to transmit time varying voltage signals to the DSO100 where they are sampled and digitized. Auxiliary input ports 115provide alternative inputs or trigger inputs.

FIG. 2 is a detailed view of the display 110. It comprises a signal plotregion 116 in which the detected voltage of a sampled signal isconventionally plotted on a vertical axis against an horizontal timeaxis. Status portions 118 of the display 110 identify the displaycharacteristics, horizontal/vertical scale, and the oscilloscope'scurrent settings. A legend region 120 identifies the mapping between thecolors used in display and the plotted signals.

FIG. 3 shows a display in which information is presented according tothe principles of the present invention. The signal plot region 116 ofdisplay 110 has a primary measurement display portion 120 in which aprimary measurement of the signal under analysis is displayed as in FIG.2. In the most common example, the primary measurement display portion120 will show the voltage level of the signal as a function of time onthe horizontal axis. According to the invention, the signal plot region116 of the display 110 also has a derived parameter display portion 122in which parameters derived from the primary measurements are plotted,also as a function of time. In the preferred embodiment, the horizontalaxis for the plotted primary measurement and the derived parameter arethe same. This characteristic is illustrated by the dotted linesextending between the waveform of the primary measurement and thederived parameter plot. In this way the plots have a common time axis sothat an operator, noticing an anomaly by reference to the derivedparameter plot can trace the location of the event to the relevantsegment of the primary measurement waveform.

In the illustrated example, the derived parameter is the time for whichthe data of the primary measurement either exceeds a threshold TH,illustrating width jitter. As a result, whereas the vertical axis in theprimary measurement display portion 122 is voltage, the vertical axis inthe derived parameter display portion 122 is time or signal width. Asalso illustrated, a closely related measurement is the time that thesignal falls below the threshold TH. This alternative parameter plot isindicated by reference numeral 122′.

FIG. 4 is a process diagram illustrating the method for generating thedisplay of FIG. 3 in which the derived parameters are based upon thelevel of the primary measurements relative to some threshold. In thefirst step 210, the primary measurements are made of the signal. Thenthe data from the primary measurement is displayed as a function of timein step 212. This yields the primary measurement display portion 120.According to the invention, the data from the primary measurements arealso compared to the threshold TH in step 214. The derived parameter,i.e., the length of time that the signal exceeds a threshold for everytransition through the threshold TH, is also plotted in step 216,yielding the derived parameter display portion 122. The derivedparameter display portion 122 preferably shares a time axis with theprimary measurements. The vertical axis, however, represents the timethe signal exceeds the threshold rather than voltage as in the primarymeasurement display portion 120.

Preferred embodiments, non-linear interpolation techniques are used suchas (sin x)/x and cubic interpolation to enhance the accuracy by whichthreshold crossings are located in time. Additionally, the use ofhysteresis values for threshold crossings and digital low pass filteringare also important to provide better noise immunity in the parametercalculations.

FIG. 5 shows another exemplary display of derived parameters withprimary measurements, according to the present invention. In this case,as in FIG. 3, the primary measurement portion 120 of the display 110shows the voltage of the signal as a function of time. The period ofthis signal, however, is derived from the data of the primarymeasurement. The derived parameter portion 122 of the display 122 thenplots the period for each cycle of the signal. This display is helpfulto show the progression of the period over time, allowing the operatorto relate the plot of the period to the actual waveform of the primarymeasurement because of the common time axis. As illustrated by derivedparameter display 122′, in alternative or complimentary embodiments, theperiod can be derived beginning at the falling portion of the signalrather than the rising portion as shown in portion 122.

FIG. 6 is a process diagram illustrating the method for generating thedisplay 110 shown in FIG. 5. As discussed in reference to FIG. 4, theprimary measurements of the signal are made and displayed in steps 210and 212. According to the present process, cycles of the waveform of theprimary measurements are then identified in step 218 by reference to thedata from the primary measurements. Based upon the identification of thecycles and the data, derived parameters are calculated, i.e., theperiods of successive cycles, and displayed as a function of time instep 220.

FIG. 7 shows another display 110 that is also based upon the cycles ofthe data from the primary measurements. In this case, the cycle-to-cyclejitter is plotted on the derived parameter portion 122 of the display110. Specifically, the vertical axis of the derived parameter portion ofthe display 122 represents the difference between the current cycleperiod and the previous cycle period (P_(n)−P_(n−1)).

In alternative embodiments, the derived parameters are based on othercycle parameters such as: the time between the maxima and minima foreach cycle, the time between minima and a previous minima for eachcycle, the time of a local minima for each cycle, the time of a localmaxima for each cycle, the time between local maxima and local minimafor each cycle, the frequency of each cycle, the rise time for eachcycle, the fall time for each cycle, the overshoot for each cycle, thepre-shoot for each cycle, and the change in pulse-width for each cyclerelative to adjacent cycles.

Especially when searching for minima and maxima in the primarymeasurements to generate the parameters, the use of hysteresis values isa useful technique for locating the events of interest. Moreover,non-linear interpolation techniques are also preferably used such as(sin x)/x and cubic interpolation to enhance the accuracy by which peaksor other events are located in time and amplitude.

FIG. 8 shows still another display of a derived parameter plot with theprimary measurement plot. In this example, the derived parameter isbased upon the period of each cycle P_(n) and the duty cycle width W_(N)or the time for which the voltage of the signal exceeds some thresholdTH. The plot of the derived parameters 122 has a vertical axis that isthe duty cycle W_(N) for a cycle divided by the period for that cycleP_(N)

The alternative derived parameter plot 122′ illustrates that theparameters can also be based on the time each cycle is below thethreshold TH.

FIG. 9 is a process diagram illustrating the technique for generating acycle time and threshold-based derived parameter display. Specifically,as in FIG. 6, the primary measurements are made, the plot of the primarymeasurements is displayed, and the cycles of the primary measurementsare identified in steps 210, 212, 218. In each cycle, the data from theprimary measurements is also compared to a threshold TH in step 222. Theparameters are then calculated and displayed as a function of time instep 224 based on the identified cycles in the signal and thethresholding of the data from the primary measurements.

In additional implementations, derived parameter displays based upon thepeak variation in the amplitude and/or the duty factor are also plottedwith the voltage of the primary measurements.

FIG. 10 shows still another embodiment of the plot portion 116 ofdisplay 110 according to the present invention. In this case the primarymeasurement portion of the display 120 plots the data from the primarymeasurements as a function of time as described previously. Cycles inthe signal are identified and then compared against an internal orexternal time reference. This can either be an absolute time referenceor a reference clock 124, as illustrated. The derived parameter portionof the display 122 in the illustrated example is a display of theinterval error jitter. This is the difference between the period of themeasured signal and the period of the reference clock edges 124. Thus,the vertical axis of the derived parameter plot 122 represents thecomparison of these two signals or the signal of the primary measurementagainst an absolute time reference in different implementations.

FIG. 11 is a process diagram showing the method for generating thedisplay as shown in FIG. 10. As in the process diagram of FIG. 9, theprimary measurements are made, the data from the primary measurements isplotted and displayed, and cycles of the primary measurements of thesignal is identified in steps 210, 212, and 218. Further, the cycles ofthe primary measurement are compared against an external time base instep 226. This can be an external clock source, or an absolute timereference either generated externally or within the oscilloscope. Thenthe derived parameters are plotted as a function of time. These derivedparameters are based upon the comparison of the cycles of the signal andthe time base in step 228.

In other implementations, the derived parameters can be based on thephase differences between the cycles of the signal and the absolute timereference, the timing errors between the cycles of the signal and theabsolute time reference, phase differences between the cycles of themeasured signal and the reference clock, and timing errors between thereference clock and the cycles of the measured signal.

FIG. 12 is a block diagram showing the internal organization of theoscilloscope 100 of the present invention. Specifically, the four ports114A-114B provide inputs to four parallel channels 130A-130D of theoscilloscope. Each channel 130 has an amplifier 132 for providing a highimpedance input to the channel. The output to the amplifier goes to asample-and-hold circuit 134 that temporarily freezes the signal fordigitization by analog-to-digital converter ADC1-4 136. The digitaloutput of the analog digital converters, 8 bits wide, in one embodiment,is stored to waveform memories 1-4 138. These waveform memoriestypically have from 1 to 16 million, eight bit deep, storage locations.Trigger 142 tracks the output from the amplifiers 132 of the channels130A-130D to search for a trigger condition. When the trigger conditionis found, the time base 140 is activated, which synchronizes theoperation of the sample-and-hold circuits 134, analog digital converters136, and waveform memories 138.

In one implementation, the sample-and-holds 134, analog-to-digitalconverters 138, and waveform memories 138 continuously freeze, digitize,and store the data that is descriptive of the voltage of the signalstransmitted to ports 114A-114B by the probes when the trigger 142 ismerely armed. The waveform memories 138 are addressed in the form of acircular buffer. Only after the trigger condition is found does thetrigger 142 and time base 140 hold the contents of the waveform memoryand thereby sample the waveform at the triggering event.

Central processing unit (CPU) 150 controls the overall operation of theoscilloscope 100. Specifically, data captured in the waveform memories138 of the channels 130A-130B are transferred by the CPU 115 into slotsin a local memory 152 via bus 148. In the preferred embodiment there areeight slots #1-#8 in the local memory 152. This allows the oscilloscopeto save up to eight separate events captured by the waveform memories138. For example, the four channels could be operated to simultaneouslysample four signals. The CPU 150 transfers their contents to slots 1through 4 of the local memory 152. Thereafter, the channels 130A-130Bwould be free to capture up to 4 more additional signal events, storingthem in slots 5 through 8 of the local memory 152 before any overwritingis necessitated.

Under operator control, the CPU 150 selects the data in local memoryslots 1 through 8 for processing and transfer to a video frame buffer154. Specifically under operator control, the data in slots 1 and 5could be selected for display. This data is then processed usingnon-linear interpolation techniques, sin(x)/x or cubic for example, andthe resulting display data transferred to video buffer 154 whichprovides the data to display 110.

FIG. 13 is a schematic diagram illustrating the mapping of the data inthe slots of the local memory 152 to the display 110. As the CPU 150operates on the primary measurement data in any one of slots 1 through 8in the local memory 152, the CPU stores the derived parameters back to avacant slot. In this way, the derived parameters utilize a slot in thelocal memory 152 that could otherwise be used by the channels to performanother signal acquisition. Briefly, as described in reference to FIGS.4 through 11, the CPU identifies cycles in the data of the primarymeasurements, thresholds the primary measurements possibly usingnon-linear interpolation to improve accuracy and hysteresis/low passfiltering for noise immunity, and/or other operations to generate thederived parameters.

One advantage of using the storage oscilloscope architecture is the factthat multiple parameters can be calculated from the same primarymeasurement data and stored for simultaneous display, for example.Moreover, parameter recalculation is possible where the original appliedthreshold, for example, used to generate the parameters is modifiedafter observation of the parameter display.

FIG. 13 illustrates an example situation in which a triangular waveformis stored in slot 1, which was generated from signal acquisition. Thedata in the array of slot 1 is indicated by reference numeral 157. Thesecond array from slot 7 is indicated by reference numeral 155 holdingparameter derived from the data in array 157. In the illustratedexample, the derived parameters relate to the period of the triangularsignal stored in array 155 of slot 1. The dotted lines 156 extendingbetween the two arrays illustrate that the data for the primarymeasurements and the derived parameters are stored with analogoustemporal organizations. For example, array location n in the array 155holds parameters derived from data in array location n in array 155.

In the preferred embodiment, array 155 holding the derived parameters ispacked with the period-based data. Each location that corresponds to thesame cycle of the primary measurements in array 154 holds the measure ofthe period in the derived parameter array. Although being somewhatwasteful in storage space, this allows the existing oscilloscope zoomand scan features to operate normally. Even when expanding orcontracting the scale using normal oscilloscope functions, thehorizontal axis of the primary measurement data and the derivedparameters remain synchronized to a common time axis.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims. Those skilled in the artwill recognize or be able to ascertain using no more than routineexperimentation, many equivalents to the specific embodiments of theinvention described specifically herein. Such equivalents are intendedto be encompassed in the scope of the claims.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A method for presenting information on a digitaloscilloscope, the method comprising: making primary measurements of asignal; displaying data from the primary measurements as a function oftime; calculating derived parameters from the data of the primarymeasurements; and displaying the derived parameters as a function oftime with the data from the primary measurements, wherein the derivedparameters are calculated and simultaneously displayed with the datafrom the primary measurements.
 2. A method as described in claim 1,wherein the step of making the primary measurements of the signalcomprises detecting a voltage of the signal at time intervals.
 3. Amethod as described in claim 2, wherein the steps of displaying theprimary measurement data and the derived parameters comprises plottingthe detected voltages and derived parameters along a horizontal timeaxis.
 4. A method as described in claim 1, further comprising displayingthe derived parameters and the primary measurement data on a commondisplay with a common time axis.
 5. A method as described in claim 1,wherein the step of deriving the parameters comprises comparing the datafrom the primary measurements to a threshold.
 6. A method as describedin claim 5, wherein the step of deriving the parameters furthercomprises determining times for which the primary measurement dataexceed the threshold.
 7. A method as described in claim 5, wherein thestep of deriving parameters further comprises determining times forwhich the primary measurement data fall below the threshold.
 8. A methodas described in claim 1, wherein the step of deriving the parameterscomprises: identifying cycles in the signal; and calculating parametersfor the cycles.
 9. A method as described in claim 8, wherein the step ofderiving the parameters further comprises determining the time betweenmaxima and minima for each cycle.
 10. A method as described in claim 8,wherein the step of deriving the parameters further comprisesdetermining the time between minima and previous minima for each cycle.11. A method as described in claim 8, wherein the step of deriving theparameters further comprises determining the time of a local minima foreach cycle.
 12. A method as described in claim 8, wherein the step ofderiving the parameters further comprises determining the time of alocal maxima for each cycle.
 13. A method as described in claim 8,wherein the step of deriving the parameters further comprisesdetermining the time between local maxima and local minima for eachcycle.
 14. A method as described in claim 8, wherein the step ofderiving the parameters further comprises determining a period for eachcycle.
 15. A method as described in claim 8, wherein the step ofderiving the parameters further comprises determining a frequency foreach cycle.
 16. A method as described in claim 8, wherein the step ofderiving the parameters further comprises determining rise times foreach cycle.
 17. A method as described in claim 8, wherein the step ofderiving the parameters further comprises determining fall times foreach cycle.
 18. A method as described in claim 8, wherein the step ofderiving the parameters further comprises determining overshoot for eachcycle.
 19. A method as described in claim 8, wherein the step ofderiving the parameters further comprises determining preshoot for eachcycle.
 20. A method as described in claim 8, wherein the step ofderiving the parameters further comprises determining a change in periodin cycles relative to adjacent cycles.
 21. A method as described inclaim 8, wherein the step of deriving the parameters further comprisesdetermining a change in pulse width for cycles relative to adjacentcycles.
 22. A method as described in claim 1, wherein the step ofderiving the parameters comprises: identifying cycles in the signal; andcomparing the primary measurements in each cycle to a threshold.
 23. Amethod as described in claim 1, wherein the step of calculating thederived parameters comprises: identifying cycles in the signal; andcomparing the primary measurement data to an absolute time reference.24. A method as described in claim 1, wherein the step of deriving theparameters comprises comparing the primary measurements of the signal toa reference clock.
 25. A method as described in claim 24, wherein thestep of deriving the parameters further comprises determining phasedifferences between the reference clock and the cycles of the signal.26. A method of operation for a digital oscilloscope, the methodcomprising: repeatedly sampling and digitizing a signal at predeterminedtime intervals and storing data generated by the digitization; derivingparameters as a function of time from the data of the primarymeasurements; and displaying the derived parameters as a function oftime, wherein the derived parameters arc calculated and simultaneouslydisplayed with the data from the primary measurements.
 27. A method asdescribed in claim 26, wherein the step of sampling and digitizing thesignal comprises detecting a voltage of the signal.
 28. A method asdescribed in claim 27, wherein the step of displaying the parameterscomprises plotting the parameters along an horizontal time axis of adisplay.
 29. A method as described in claim 28, further comprisingdisplaying the parameters with the data from the digitization on acommon display with a common time axis.
 30. A digital oscilloscope,comprising: at least one digitization channel that performs primarymeasurements of a signal and generates data indicative of themeasurements; a data processing unit that derives parameters as afunction of time from the primary measurement data; and a display onwhich the data from the primary measurements and the derived parametersare plotted as a function of time, wherein the derived parameters arecalculated and simultaneously displayed with the data from the primarymeasurements.
 31. A digital oscilloscope as described in claim 30,further comprising at least four digitization channels.
 32. A digitaloscilloscope as described in claim 30, wherein the at least onedigitization channel comprises: a sample-and-hold circuit that freezesthe signal; an analog-to-digital converter that digitizes the signalfrom the sample-and-hold circuit; and a waveform memory that stores thedigital data generated by the analog-to-digital converter.
 33. A digitaloscilloscope as described in claim 30, wherein the data processing unitcontrols the display to plot the data from the primary measurements andthe derived parameters along an horizontal time axis.
 34. A digitaloscilloscope as described in claim 30, wherein the data processing unitcontrols the display to plot the derived parameters and the data fromthe primary measurements with a common time axis.
 35. A method forpresenting information on a digital oscilloscope, the method comprising:making primary measurements of a signal; displaying data from theprimary measurements as a function of time; multiple arrays of differentparameters from the data of the primary measurements; and calculatingand simultaneously displaying the different derived parameters as afunction of time with the data from the primary measurements.
 36. Amethod for presenting information on a digital oscilloscope, the methodcomprising: making primary measurements of a signal; displaying datafrom the primary measurements as a function of time; deriving parametersfrom the data of the primary measurements; and calculating andsimultaneously displaying the derived parameters as a function of timewith the data from the primary measurements; recalculating the parameterbased on new criteria after observing the display of the parameters.